When I was in high school decades ago, I did 15 minutes of less of test prep, which was trying the practice questions in the booklet that included the sign-up sheet.
But for someone who wants to do more test prep, the best way to start is to try an old released test under test conditions (time limits per section, etc.), grade it, and then use the results to find out what type of questions are the ones the test taker needs to focus test prep on.
For some students, it may be worth trying both the ACT and SAT old released tests, since some students do much better on one than the other.
Generally, I would say that a reasonable testing schedule for a student who will apply to colleges using the SAT or ACT would be:
- 11th grade, fall: PSAT if NM or CBNRP is a possibility.
- 11th grade, spring: SAT and ACT, or just whichever one was better if practice testing has already determined one to be better for the student
- 12th grade, fall: optional retry of SAT or ACT (whichever was better initially, after any additional test prep if desired)